Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration to determine the properties of earth's subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to a surface 14 of the ocean. The streamers may be disposed to have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downward toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on the streamer 12. Based on the data collected by the receiver 11, an image of the subsurface is generated by further analyses of the collected data.
The seismic source array 16 includes plural individual source elements. The individual source elements may be distributed in various patterns, e.g., circular, linear, at various depths in the water. FIG. 2 shows a vessel 40 towing two wide tow lines 42 provided at respective ends with paravanes 44. A paravane is a structure that when towed underwater or at the surface of the water provides a lift as will be discussed later. Plural lead-in cables 46 are connected to streamers 50. The plural lead-in cables 46 also connect to the vessel 40. The streamers 50 are maintained at desired separations from each other by separation ropes or spur lines 48. Plural individual source elements 52 are also connected to the vessel 40 and to the lead-in cables 46 via ropes or cables 54.
The paravanes 44 produce a lift, created by their motion through the water, that stretches the spur line 48 so that the streamers 50 are pulled outwardly to maintain their separation relative to the vessel path during the seismic survey. FIG. 3A shows a paravane 44 connected with straps 60 and 62 to a connecting device 64. More than two straps may be used to connect the paravane to the connecting device, for example six straps. The connecting device 64 is also connected to the wide tow line 42 and the spur line 48.
When moving in water, a force 70a appears on the paravane 44, and this force may be decomposed in a drag force 72, which acts as a brake on the towing vessel and a lift 74 that is applied to the separation rope 48 to maintain the streamers 50 separated from each other. To balance this force an equal and opposite force 70 needs to be applied to the connecting device 64 for towing the paravane 44. Corresponding components are present on the connecting device 64, i.e., paravane drag 72a and lift 74a. It is noted that the paravane drag 72a and lift 74a are projections of the total force 70 on the spur line 48 and the wide tow line 42. As the spur line 48 and wide tow line 42 are not perpendicular to each other, lift force 74a is not equal to lift force 74. In other words, the force 70a is decomposed along perpendicular axes A1 and A2 while the opposite force 70 is decomposed on axes B1 and B2, which are not perpendicular to each other.
FIG. 3B shows the same arrangement when an angle between the separation rope 48 and the wide tow line 42 is increased from α1 to α2. It is noted that an increase in the angle between the separation rope 48 and the wide tow line 42 is beneficial as a component of the lift 74a along the separation rope 48 is increased and a force component 76 on the wide tow line is decreased. In other words, total lift is redistributed/transferred from the wide tow line to the separation rope. Thus, force for maintaining the streamers 50 separated from each other is increased, which is desirable for a seismic survey when increasing the size of the spread.
However, due to the hydrodynamic tension 80 acting on the wide tow line 42 while being towed in water, there is a maximum angle between the wide tow line and the separation rope that may be achieved for a given towing system. FIG. 4 shows different wide tow lines/separation ropes configurations 90, 92 and 94 having corresponding angles α1, α2 and α3 with α1<α2<α3. It is noted that a next configuration 96 has an angle α4 equal to the angle α3 of configuration 94. This means that α3 is a critical angle, i.e., it is not possible to further increase the angle between the separation rope and the wide tow line using conventional towing systems.
FIG. 5 shows a load-sharing tension between the spur line and the wide tow line for various angles. It is noted that the critical angle α3 is reached when tension in the separation rope (curve 100) is not at maximum. Curve 102 indicates the tension in the wide tow line.
Thus, it is desirable to find a way to increase the critical angle for a given towing system so that more lift is available in the separation rope for maintaining the streamers or sources at predetermined positions even if water currents or other factors are present. Accordingly, it would be desirable to provide systems and methods that provide a towing system with an increased critical angle.